Can Vertex Paint Normalize Color Channels?
I've managed to script myself a solution:

import bpy
import random
import math
from mathutils import Color, Vector

bl_info = {
  "name": "Vertex Color Normalizer",
  "description": "Allows the vertex colors of a mesh to be normalized (useful if colors control texture splatting).",
  "author": "Markus Ewald",
  "version": (1, 0),
  "blender": (2, 65, 0),
  "location": "View3D &gt; Add &gt; Mesh",
  "warning": "", # used for warning icon and text in addons panel
  "wiki_url": "http://wiki.blender.org/index.php/Extensions:2.5/Py/"
              "Scripts/VertexColorNormalizer",
  "category": "Paint"
}

class VertexColorNormalizer(bpy.types.Operator):
    bl_idname = "object.normalize_vertex_colors"
    bl_label = "Normalize Vertex Colors"
    bl_description = "Normalize vertex colors to enable additive texture splatting"

    def execute(self, context):

        # What I want to do:
        # - Iterate over the colors of all the vertices in active mesh
        # - Normalize each color (divide by sqrt(r *r + g * g + b * b))
        # - Assign it back to the vertex

        for i in context.active_object.data.vertex_colors[0].data:   
            r = i.color[0]
            g = i.color[1]
            b = i.color[2]

            magnitude = math.sqrt(r * r + g * g + b * b)
            if magnitude &gt; 1:
                normalized_color = (r / magnitude, g / magnitude, b / magnitude)
                i.color = Color((normalized_color))

        return {'FINISHED'}

def register():   
    bpy.utils.register_class(VertexColorNormalizer)   

if __name__ == "__main__":   
    register()  



Save as C:\Users\&lt;Login&gt;\AppData\Roaming\Blender Foundation\Blender\2.72\scripts\addons\NormalizeVertexColors.py
Start Blender, activate Preferences, Addons, Paint, "Vertex Color Normalizer"
Select target mesh, spacebar, type 'normalize' and pick the "Normalize Vertex Colors" entry


My knowledge of Python or the Blender object model are too limited to let this happen automatically when painting vertices, but it's better than nothing, I guess :)